The Single Clue

by John W. Campbell, Jr. (pages 89-93 of December 1936's Astounding Stories)

NO ASTRONOMER ever studies stars, planets or other heavenly bodies.
Most astronomers believe they never will, though some hope eventually to be able to study other planets.
There is a general misunderstanding of the life study of astronomers; a bit of thought will remind one, however, that since no astronomer has ever reached Mars, none can have studied it.
The astronomer spends his life studying light, radiation, and the characteristics of radiation.

All astronomical knowledge possessed by man to-day has been obtained solely by the study of the single clue available to mankind: extraterrestrial light compared with terrestrial light sources.
There is, perhaps, a very minor and unimportant exception to that: meteors and meteorites (a meteorite is a meteor that reaches Earth's surface) have yielded confirmatory data.
But even that is largely misleading, since meteorites all contain enormously higher percentages of either silicon or iron than typical specimens of universal matter would.
Present knowledge indicates that instead of iron or silicon dominating the universe, as meteors indicate, hydrogen probably constitutes at least one third and probably on half of all matter.

Thus, of necessity, an astronomer is actually a student of light, its properties, and observable phenomena of light.

The next planet to be considered in this series on the solar system is Jupiter.
Jupiter differs so widely from the previously considered planets; Jupiter has done so much for this study of light, and, in turn, the study of light has yielded so much data regarding the planet, that it is imperative we understand better what and how we study, than that we simply quote facts.

The Greeks were not great experimental scientists; they preferred the domain of logic to actual experimental proof.
But the Greeks did make some highly ingenious experiments, and their logic was generally sound.
They had already developed two opposing theories of planetary motion: the stationary Earth circled by stars, Sun, Moon and planets; and the stationary Sun circled by the planets and the rotating Earth.

As an excellent example of their experimental ability, the Greeks measured the speed of sound thousands of years ago, and obtained highly creditable results.
Modern methods depend on wave length of sound, and the vibration period of a column of air over water, or some variation of that; a highly technical, though highly accurate method.
The forthright Greeks did it almost equally well by having two men stationed on hilltops, each equipped with noise makers.
A signaled to B and B replied to A.
A measured the time elapsed between the time of his signal and B's reply.
Then, knowing the distance A to B to A, and that time, they readily got the speed of sound.

NATURALLY, being interested in light, they tried the same scheme with it, using torches, or heliograph mirrors.
Also, naturally, they failed to detect any elapsed time, since the twenty-mile journey took light so infinitesimal a fraction of a second.
But that by no means meant that they had learned nothing, they had.
They recognized it by adding to the first law of light -- "Light travels in straight lines" -- a second law of light: "Light travels at immense (infinite?) speed."

To understand the importance of this remember that Kepler worked entirely from Tycho Brahe's observed results working backward from results to a theory which would account for those results.
It was Tycho Brahe's extreme accuracy that made Kepler go to the elliptical orbits rather than the old circular-orbit theory, and derive the immensely important Kepler laws.
In turn, Newton's laws of gravity were based on Kepler's discoveries.
All worked back, then, to the accuracy of Tycho Brahe's observations.

But Earth moves in an orbit 186,000,000 miles in diameter.
Thus it takes light almost exactly 1,000 seconds to cross that orbit at the actual speed of 186,000 miles a second.
In observing Mars position then -- since Mars is sometimes about 50,000,000 miles from Earth and sometimes about 230,000,000 miles distant -- Mars is never where we see it, due to the time light spends in crossing that gulf of space; it is always at least 50,000,000 divided by 186,000 seconds away from where we see it; and Mars travels at a speed of many miles a second!

The error is never less than that, and at times rises to 230,000,000 divided by 186,000.
But Mars is large, and even that distance of motion is unimportant, so Kepler got the right answers.
But -- if light traveled only 186 miles a second, the results would have been utterly unintelligible.
More than a million and a quarter seconds would have been required when Mars was on the far side of the Sun; in other words, Tycho's observations would have shown Mars where it had actually been more than two weeks before!

But the ancient Greek experiments had shown that light did not travel so slowly, so Kepler could go ahead with confidence.
Knowledge of this property of light, high speed of travel, formed the basis of the greatest single discovery of all time to that date; Newton's gravity.

And the straight-line-travel knowledge made Newton's gravitational theory an immensely powerful tool for analyzing light.
For straight line travel made light a good indicator of position, which sound, capable of rounding corners, is not.
From position and change of position Newton's gravity theory was able to make light reveal something it did not intrinsically show at that time: the mass of the radiating body.

IT WAS Kepler's accuracy, based on Tycho Brahe's accurate observations, that made the elliptical-orbit theory partially acceptable.
But the most ancient objection, and the most weighty with logical men, was still to be overcome.

All experience had definitely shown that unsupported bodies fall.
Kepler was suggesting that unsupported planets, whole worlds, floated in nothingness on nothing at all.
And an even huger Sun floated on less, if anything.
It was obviously illogical.

Don't think they were foolish.
They were perfectly right; it was completely illogical.
All former experience denied it; that Kepler thought of it at all shows not their hard-headed reactionism, but his flight of genius.
They denied the theory because it went against all known fact, and theories that do that are wrong, and we so regard them today.

In the very early part of the 1600s -- the greatest century astronomy had ever known -- Jan Lippershey's children were also illogical.
It was an anciently known fact that a lens, while capable of magnifying things close at hand, blurred things at a distance.
Two lenses made things worse -- experimental fact which can be verified by any one.
The telescope was impossible.

Jan Lippershey was a Dutch eyeglass grinder, and his iniquitous, inquisitive children played with their father's hard-made[hand-made?] lenses.
The recently invented concave lenses for short-sighted people were one of Jan's specialties.
And his illogical children, not knowing that telescopes were impossible, modified the ancient third law of light -- "Light may be refracted, or bent, from its straight-line course by any transparent medium" -- to the extent of adding that if a concave lens is held near the eye, and a convex lens at a little distance from the eye distant things seem near.

That was the invention of the telescope, and with this added knowledge of the handling and the properties of light, astronomy took a great step; the Kepler theory was established for all time.
Galileo looked at Jupiter through his crude little telescope -- of the type we would call opera glasses -- and saw the four giant moons of Jupiter.

He couldn't say how an unsupported body could float in space, but neither could the critics say any longer, "All previous fact indicates that unsupported bodies fall."
The four giant, unsupported moons of Jupiter didn't.
They could watch and observe a miniature solar system in action, with Jupiter as the sun, and the four moons as planets.

That was 1610.
Naturally an immediate, vast interest was roused in these moons.
They are so large, and so bright that they would be readily visible to the unaided eye, but Jupiter is so much larger, and so far brighter that his brilliant rays drown out the moons.
But almost anything in the way of optical aid, even so inefficient as an opera glass, will make them readily visible.
To-day a ten-cent-store telescope will probably be more powerful and more efficient than Galileo's triumph of optical science.

Not because we are smarter -- simply because we know all the things Galileo learned before he died, and many, many things equally hard-working men learned and passed on to us.

The discovery of these moons meant that many observers watched them, and many turned the wonderful new telescopes on every other object in the skies.
There were countless thousands of new discoveries -- undreamed-of stars, beyond the range of naked-eye observation; double stars where one had been thought to be; mountains on the Moon; the phases of Venus and Mercury.
For the first time men could readily see that Venus had a full, new, quarter and half stages.
Ah, it was a wonderful time for astronomers!

AND ONE ROEMER, a Danish astronomer, in 1675, calculated the orbits of those four giant moons.
First, of course, came a long a laborious period of observation, conducted when Jupiter was nearest the Earth, and visibility best.
Fortunately, his task was made somewhat easier by the fact that the moons frequently went into total eclipse in the shadow of Jupiter's immense bulk.
This eased things, because the inadequate telescopes of the day made it hard to tell exactly how many degrees along in the orbit it was -- but when it winked out, you knew accurately.

Then six long months of wearisome calculation was needed before the results were finished.
Roemer had to wait anyway, to let any possible errors mount up till they would show, by adding in each swing around Jupiter.
But at last, he again observed the moons, and timed those eclipses and -- his calculations were six hundred seconds off.

Roemer knew he could not be that far off.
Some hitherto unguessed factor had entered.
It was like calculating the distance from New York to San Francisco in inches, and finding your result six miles wrong.

Roemer announced presently that the orbits of the moons of Jupiter were this and so, and that and such, and furthermore that the speed of light was immense, and was not infinite.
It was about 180,000 miles a second.
For the first time men had found a signalman far enough away to measure the speed of light!

And while they were learning to pin down the speed of light somewhere between "immense" and "infinite," which had been a hotly debated point for some twenty centuries, they learned another law of light, and cursed it with a heartiness and depth that would have left the sailormen of the time in awe.
For the fourth law of light was: white light, refracted or bent by a lens, is broken up into colored images, because violet light is bent more than blue, blue more than green, and so on.

It meant to the astronomers that their telescope lenses were limited in size; a big lens was fine, but it produced images that looked like a water-color painting after a cloudburst.
Astronomers had sighed, and turned to mirrors which did not have this failing -- but plenty of others, nevertheless -- as the only hope for larger telescopes.

The sixteen hundreds; the heyday of astronomy!
Kepler's elliptical orbits in 1610, or so, the telescope about the same time, the moons of Jupiter almost simultaneously; then Newton was born in 1643.

In 1680 -- approximately -- the laws of motion were announced, in connection with the laws of gravity in Newton's Principia (The Mathematical Principles of Natural Philosophy).
Nearly two centuries were to pass before any discovery of equal importance was to be made, one that could even challenge the vast scope of the law of gravity in its scientific implications.
Gravity is a principle so completely fundamental that it can never, in all time to come, be forgotten.
Telescopes may some day use no lenses; radio may be outmoded; but gravity is a fundamental and forever-important law.

IN 1666 the hated fourth law of light attracted Newton's attention, and he tried an experiment to prove that white light is a blend of colored light.
He admitted sunlight through a round hole to a prism, getting then the familiar colors, ranging smoothly, gradually, featurelessly, from violet through blue, green, yellow, orange to red.
By means of a second prism he showed that they could be recombined to a beam of white light.
Newton proved white light was compounded of colored.
It was a great discovery.

For the fourth law of light is the law of the spectroscope.
By it, to-day, the secret language of light may be read; by it, light talks like a garrulous old maid at a gossip's tea party.
It tells all the secrets of the universe.
By it we can analyze the Sun and the million-billion-mile-distant star; we sample the air of Jupiter and Mars; and we time the speed of the moving stars.
By it we analyze the minerals of Earth or star.

In 1666 America was a howling wilderness, where Puritan Pilgrims held on by tooth and toenail to a narrow strip of seacoast.
England had just overthrown Cromwell.
Men sought unicorns for their magic, cure-all horns.
Oxygen was not to be dreamed of for a century and more.
Chemistry, the basis of modern civilization, was alchemy, and men sought the philosophers' stone.

In 1666 Newton, the man who developed the law of gravity from idle speculation on a falling apple, used a round opening to produce his spectrum, and got round images of the Sun in every color, smoothly overlapping and featureless.
A spectroscope uses exactly the same apparatus save that they have a thin, hairline slit, so that each color is thrown in a hairline, sharply distinguishable mark of light.

Literally, by a hairline Newton missed the spectroscope.
Had he used a slit, the spectrum of the Sun would have been bright colors crossed by mysterious black bands and lines.
He could not have left that mystery untouched.
He would have found that sodium thrown on a candlewick would produce bright-yellow lines matching exactly two powerful dark lines in the mysterious solar spectrum.
Calcium would have given him red lines, copper and other metals --

Chemistry would have started up like a stung rabbit from spectroscopy, not test tubes!
Oxygen in a year, not a century and a half.
The elements of the rocks in months.

But spectroscopy waited untouched from 1666 to 1802.
Can you conceive what an alien world this might have been had a man who mastered gravity, calculus and the laws of motion used that slit, the one great thing that challenges gravity for supremacy in teaching mankind?